Lithium secondary batteries, which are recently in the spotlight as a power source of portable and small electronic devices, may exhibit high discharge voltages that are two times or more than those of batteries using a typical alkaline aqueous solution by using an organic electrolyte solution. Thus, the lithium secondary batteries exhibit high energy density.
Graphite is mainly used as an anode active material of the lithium secondary battery. However, graphite has a low capacity per unit mass of 372 mAh/g and a high-capacity lithium secondary battery may be difficult to be prepared by using graphite.
As an anode active material exhibiting higher capacity than graphite, a material forming an intermetallic compound with lithium, such as silicon, tin, and an oxide thereof, may be promising. However, volumes of the above materials may expand because crystal structures thereof may be changed when absorbing and storing lithium. When silicon absorbs and stores the maximum amount of lithium, the silicon may be transformed into Li4.4Si and the volume of Li4.4Si may expand due to charging. With respect to the rate of increase in volume due to the charging, the volume may expand up to about 4.12 times the volume of the silicon before the volume expansion.
Therefore, a significant amount of research into an increase in the capacity of an anode active material, such as silicon, i.e., a decrease in a volume expansion coefficient by alloying of silicon, has been conducted. However, since a metal, such as silicon (Si), tin (Sn), and aluminum (Al), is alloyed with lithium during charge and discharge, volume expansion and contraction may occur. Thus, cycle characteristics of the battery may degrade.
Although silicon is known as an element that may most likely provide high capacity, it may be very difficult to amorphize silicon itself alone and it may be also difficult to amorphize an alloy including silicon as a main component. However, a method of easily amorphizing a silicon-based material has recently been developed by using mechanical alloying.
For example, as a method of preparing an anode active material for a lithium secondary battery using a silicon alloy, a method of preparing an anode active material has been developed, in which powders of a silicon element and an element M (where M is nickel (Ni), cobalt (Co), boron (B), chromium (Cr), copper (Cu), iron (Fe), manganese (Mn), titanium (Ti), or yttrium (Y)) are alloyed by mechanical alloying to form a SiM alloy, the SiM alloy is heat treated, and the heat-treated SiM alloy is then alloyed with powder of an element X (where X is silver (Ag), copper (Cu), and gold (Au)) by mechanical alloying to obtain a SiMX alloy.
However, with respect to the anode active material for a lithium secondary battery prepared by the above method, its charge and discharge capacity may be decreased due to the degradation of silicon as charge and discharge cycles proceed. With respect to the mechanical alloying, since the destruction of an alloy structure may occur due to the intercalation and deintercalation of lithium, the cycle characteristics may degrade.
Therefore, there is a need to develop an anode active material which may replace a typical anode active material and may improve discharge capacity, efficiency, and lifetime characteristics when used in the lithium secondary battery.